Abstract: This paper describes the analysis of a MCS long life cycle in the south of China during April 2008. Using FY-2C geostationary satellite multi-channel cloud images, radar observations, records obtained from automatic weather stations andothers irregular observations, a long life cycle mesoscale convective systems(MCS) was studied to analyze the maintainable mechanism of MCS, whichoccurred in the middle part of Guangxi on 12 April 2008, and then moved to the southeast of Guangxi,the western Guangdong and The South China Sea. The MCS lasted 25 hours, and hence may be classified as a long life cycle. Section lines of TBB were given in the hourly cloud images of water vapor (WV) on April 12, from 05:00 to 20:00 BST, to analyze the TBB character change over time. Images show that there is an apparent course from wet to dry state in upper troposphere before 14:00. Simultaneously in the lower troposphere, a mesoscale water conveyor belt which was sustained and stable and extended from Beibu Gulf to southeast of Guangxi could be detected from the composite cloud images of infrared(IR1)and near infrared　(IR4) and visible light　(VIS). In the dry area of upper air and wet area of low altitude, an instability area was formed in the southeast of Guangxi, which favored the formation and maintenance of MCS. It could be concluded from records of automatic weather stations that while the quasistationary front moving slowly from the north to the south, continuously lifting the wet warm air and triggering the deep convection, a MCS was formed. Characterized by the relationship between MCS forming course and the changes of atmospheric pressure, air temperature, wind velocity, precipitation, it could be regarded as a typical lifting and trigger course of the front. Results show that the main favorable factors maintaining a MCS long life cycle can be concluded in 2 points as below. Firstly, while the upper air changed from wet to dry, the water vapor and heat were carried from Beibu Gulf by a continuing mesoscale vapor conveyor belt in the lower troposphere to form the unstable atmospheric structure, providing an advantageous condition for MCS maintenance. Secondly, old convection cells of MCS gradually weakened in the way to the east,and were constantly replaced by new ones occurring in thesouthwest of the quasi stationary front moving slowly southwards provided MCS with energy to survive. A concept model of MCS is presented in the essay.